Optical devices and methods of manufacture of optical devices

ABSTRACT

A method for performing a multi-stage dilation of optical fibres is described, the method comprising performing successive dilation steps such that the adiabatic condition is maintained throughout the fibre. There is also described various optical devices employing such multi-stage dilated optical fibres, as well as methods of manufacture of the optical devices.

This application is a 35 U.S.C. 371 national phase filing of PCT/EP2010/063151, filed Sep. 8, 2010, which claims priority to Irish national application number S2009/0787 filed Oct. 9, 2009, the disclosures of which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention relates to optical devices and methods of manufacture thereof, in particular single-mode optical fibre devices.

BACKGROUND OF THE INVENTION

Optical fibres that guide light by total internal reflection consist of a cylindrical core that has a higher refractive index than the surrounding cladding. For single-mode operation, the core size and the index difference between the core and cladding are such that only the fundamental mode is propagated for a given spectral bandwidth, as determined by the characteristic waveguide number or V-number. (A single mode fibre is an optical fibre that is designed for the transmission of a single ray or mode of light.)

With reference to FIG. 1, an example of a single-mode optical fibre system is indicated at 10. The system 10 comprises a single-mode optical fibre 12 having an internal core 16 within external cladding 18. The core 16 and cladding 18 are protected within an external buffer 14, which is shown as being stripped along the length of the fibre that is to be processed. A cross-section of the fibre 12 across line A-A is indicated at 20. Typical dimensions for a standard telecommunications optical fibre 12 would be 9 μm diameter for the core 16 and 125 μm diameter for the cladding 18.

In the case of high-power transmission through standard single-mode fibres, end terminations of fibres and in-line splices or interconnects can introduce undesirable back-reflections and facet distortions that can lead to system damage and failure. Further complications can arise due to, e.g. dirt at the end termination and/or between the end of the fibre and an associated connector. In order to reduce this problem, it is desirable to reduce the power density by expanding the mode field diameter. This can be achieved using a variety of techniques, including fibre tapering, thermal core diffusion, lensing including bulk and grin lensing, fibre end shaping, and splicing on dissimilar fibres including e.g. multimode fibres. However, where a typical beam diameter of ˜50 μm is desired, each of these solutions has associated problems.

In the case of tapering, the fibre becomes small, difficult to handle and more sensitive to external influences—making it difficult to package. The diffusion approach is limited in the extent to which the beam may be expanded before loss becomes significant. Lensing does not reduce the optical power density at the fibre end-face, generally involves the introduction of free-space facets, back-reflections, glues, alignment issues and loss within in-line fibre pigtailed bulk-optic sub-systems, and is expensive. Using dissimilar fibres requires a splice and introduces back-reflections and loss where the beam diameter is not mode-matched, and it can be a relatively expensive process compared with the approach described here.

An alternative technique which has been recently proposed is that of fibre fattening (also referred to as fibre up-tapering or fibre dilation), discussions of which may be found in [1] PhD thesis, Elaine M. O'Brien, Lightwave Technology Research Centre, University of Limerick; [2] “Up-tapering of optical fibres using a conventional flame tapering rig”, G. Kakarantzas, L. Prill-Sempere and P. St. J. Russell, CFK2, Optical Society of America-CLEO/QELS Conference, 2007; and [3] “Adiabatic dialated standard and speciality optical fibres”, N. Healy, D. F. Murphy, E. M. O'Brien and C. D. Hussey, Poster080 Photonics Ireland 2007 (Galway), which are incorporated herein by reference in their entireties.

In known fibre fattening processes, a fibre to be fattened is positioned between a pair of holders, and a heat source is applied along a length of the fibre to soften the core and cladding material. The heat source may be a conventional flame, or could comprise an arc, laser, or other heat source. The action of heating a fibre that is subjected to a compressive force above its glass transition temperature results in the expansion of the width of the fibre in conjunction with a reduction of the fibre length.

An example of the effects of up-tapering is shown in FIG. 2, which shows fibre 12 after up-tapering has taken place. As can be seen from FIG. 2, the length of the fibre 12 has decreased, the newly-fattened fibre 12 a now showing transitions 22 between the end portions of the fibre 12 a and the expanded middle portion 100 of the fibre 12 a. An indication of the cross-section of the fibre 12 a along line B-B is indicated at 24. Typical dimensions of the expanded cross-section after up-tapering would be 30 μm diameter for the core 16 and 375 μm diameter for the cladding 18.

Such up-tapered fibres provide for numerous advantages, e.g. the reduction of optical power density, the improvement of mode-matching between spliced dissimilar fibres, and the flattening of the wavelength response of fused directional fibre couples.

The up-tapering process is limited by a number of conditions which must be satisfied:

-   -   1. The adiabatic condition needs to be satisfied—i.e. the         transition between the fattened and non-fattened sections needs         to be sufficiently smooth to ensure the launch of only the local         fundamental mode, so as to avoid any losses due to the         transition. For the transitions to be adiabatic, at any point         along the processed fibre, the transition must satisfy the         slowness criterion:

${\frac{a}{z}}\frac{a}{z_{b}}$

-   -    This is known as the adiabatic condition, wherein a is the core         radius at any position z along the transition such that da/dz         defines the taper angle and z_(b) is the beat length or period         of power oscillations between the excited modes of the system.         The shortest beat length can be considered as that between the         HE₁₁ mode (i.e. the designation for the fundamental mode of an         optical fibre) and the closest mode of the same symmetry, the         HE₁₂ mode. Transition losses due to non-compliance with the         adiabatic condition are one of the more considerable limitations         in fibre up-tapering.     -   2. A waveguide needs to be maintained. In conventional optical         fibres, light is guided by total internal reflection, which is         made possible by the index difference between the core and         cladding. In general, the cladding used is silica, and the core         has a raised index that is achieved by doping silica with         germanium. The heating of the fibre during the fattening process         results in thermal diffusion of the core dopant, germanium. With         diffusion, the index difference between the cladding and core is         reduced and the waveguide becomes weaker. Unless the diffusion         is controlled, the diffusion may occur to such a degree that         there will effectively no longer be an index step, and the         optical fibre no longer acts as a waveguide. Further, any         diffusion that does occur needs to satisfy the adiabatic         condition given in 1. above.     -   3. Physical size mismatch. As a fibre is fattened, the fattened         section becomes larger and heavier to the point that the         standard fibre leads are no longer able to support its weight,         and an inevitable sagging will take place.     -   4. Mode-area limit. Taking the example of large mode area fibres         for lasing, as the mode area is increased, the fibre's ability         to maintain single-mode only propagation is reduced, and light         couples into the other modes of the fat, and accordingly highly         multimode, structure. A number of techniques can be used to         strip out higher mode behaviour and therefore maintain         single-mode operation. This is a minimal concern in cases where         the fattened fibre section is fattened over a short length.

Accordingly, current fibre fattening techniques are limited to the expansion that can be achieved, typically up to ˜2.25 times dilation of the original fibre. It is an object of the invention to provide a new method of fibre fattening method that allows for greater dilation of fibres, while satisfying the limitations described above.

SUMMARY OF THE INVENTION

Accordingly, there is provided a fibre dilation method for providing a multi-stage dilated optical fibre, the method comprising the steps of:

-   -   performing a first-stage fibre dilation process on an optical         fibre to form a first dilated section of the optical fibre; and     -   performing a second-stage fibre dilation process on said first         dilated section of the optical fibre to form a second dilated         section of the optical fibre, wherein said second dilated         section is arranged such that the transition in diameter formed         between said first dilated section and said second dilated         section is spaced from the transition in diameter formed between         the undilated section of the optical fibre and said first         dilated section.

As the transitions between successive stages are spaced from one another, the adiabatic condition can be satisfied, and the transition losses kept within acceptable limits. The spacing is chosen to prevent significant transition losses between stages.

Preferably, said fibre fattening process comprises:

-   -   applying a heat source along a portion of the length of an         optical fibre to soften said portion of the optical fibre; and     -   applying a compressive force to said portion of the optical         fibre to dilate said portion.

Preferably, the method further comprises the steps of iteratively performing at least one successive fibre dilation process on the dilated section of a preceding stage, wherein the transition formed by said at least one successive fibre dilation process is spaced from the transition formed by the preceding stage.

As the transitions are spaced between successive stages, then significant transition losses between stages are prevented from occurring.

Preferably, the spacing is chosen such that the adiabatic condition is satisfied.

The adiabatic condition states that:

${\frac{a}{z}}\frac{a}{z_{b}}$

wherein a is the core radius at any position z along the transition in diameter such that da/dz defines the taper angle and z_(b) is the beat length between the HE₁₁ and HE₁₂ modes. HE₁₁ is the designation for the fundamental mode of the optical fibre, with HE₁₂ being the closest mode of the same symmetry.

Preferably, the spacing between successive transitions is 5 mm. This would be preferable for situations where a stationary oxy-butane flame is used as the heat source. In the case of a laser heat source being used, a smaller spacing may be preferred, due to the sharper thermal edges of the heat source. With a sophisticated tapering rig apparatus and moving a flame/laser heat source, the spacing between successive transitions could be reduced to a quasi-continuous transition or “zero-spacing”, rather than a step transition.

Preferably, said first-stage fibre dilation process comprises dilating a section of said optical fibre to 2-3 times the diameter of said optical fibre, further preferably, 2.25 times the diameter.

Preferably, said second-stage fibre dilation process comprises dilating said first dilated section of the optical fibre to approximately 4-5 times the diameter of said original optical fibre, further preferably, 4.5 times the diameter.

There is further provided a multi-stage dilated optical fibre manufactured according to the above method.

The invention further provides for a method for the low-loss coupling of standard optical fibres with large mode area optical fibres, the method comprising the steps of:

-   -   manufacturing a multi-stage dilated optical fibre;     -   cleaving said multi-stage dilated optical fibre to provide a         large mode area cleaved end; and     -   splicing said cleaved end of said multi-stage dilated optical         fibre to a large mode area optical fibre.

In general, a standard fibre is multi-stage dilated up to the point of optimum mode area matching with a large mode area fibre, e.g. a high-power fibre laser type fibre. Then the multi-stage dilated standard type fibre is cleaved at the dilated section and spliced to the large mode area fibre to form a low-loss interface between the large mode area fibre and the standard fibre through the dilation of the standard fibre. In addition or alternatively, the large mode area fibre may be tapered down to match the dilated section of the standard fibre.

Preferably, said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre.

Preferably, the method comprises the step of selecting a large mode area optical fibre such that the diameter of the core of said large mode area optical fibre substantially corresponds to the diameter of the core of said cleaved end.

The invention further provides an optical fibre comprising a portion of standard optical fibre and a portion of large mode area optical fibre coupled according to the above method.

The invention further provides for a method of manufacture of an optical wavelength converter, the method comprising the steps of:

-   -   manufacturing a multi-stage dilated optical fibre;     -   cleaving said multi-stage dilated optical fibre to provide a         first cleaved end of a multi-stage dilated optical fibre;     -   providing a large mode area doped optical fibre having a first         end and a second end;     -   coupling the core of said cleaved end of a multi-stage dilated         optical fibre with the first end of the core of said large mode         area doped optical fibre, such that said cleaved end of a         multi-stage dilated optical fibre and said large mode area doped         optical fibre form an optical wavelength converter operable to         convert optical signals of a first wavelength received at the         uncleaved, undilated end of said multi-stage dilated optical         fibre to optical signals of a second wavelength at said second         end of said large mode area doped optical fibre.

Preferably, said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre

Preferably, the method comprises the step of selecting a large mode area optical fibre such that the mode diameter of the core of said large mode area optical fibre substantially corresponds to the mode diameter of the core of said cleaved end.

The mode diameter/area is governed both by the physical dimensions of the fibre and by the index step between the core and the cladding. It is possible to have matched physical diameters but unmatched modes. To optimise the match between the modal areas, both the physical diameter and index step size should be considered.

The invention further provides an optical wavelength converter manufactured according to the above method.

The invention further provides for a method of manufacture of an optical fibre amplifier, the method comprising the steps of:

-   -   manufacturing a multi-stage dilated optical fibre;     -   cleaving said multi-stage dilated optical fibre to provide a         first and a second section of a multi-stage dilated optical         fibre having respective first and second cleaved ends and         uncleaved ends;     -   providing a large mode area doped optical fibre having a first         end and a second end;     -   coupling the core of said first and second cleaved ends of said         multi-stage dilated optical fibre sections with the core of the         respective first and second ends of said large mode area doped         optical fibre to form an optical fibre amplifier, the amplifier         operable to amplify an optical signal transmitted between said         first and second uncleaved ends through said multi-stage dilated         optical fibre sections and through said large mode area doped         optical fibre.

Preferably, wherein said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre.

The invention further provides an optical fibre amplifier manufactured according to the above method.

The invention further provides for a method of manufacture of an optical fibre laser, the method comprising the steps of:

-   -   manufacturing a multi-stage dilated optical fibre;     -   cleaving said multi-stage dilated optical fibre to provide a         first and a second section of a multi-stage dilated optical         fibre having respective first and second cleaved ends and         uncleaved ends;     -   processing said first and second cleaved ends to provide         reflecting means at said cleaved ends;     -   providing a large mode area doped optical fibre having a first         end and a second end;     -   coupling the core of said first and second cleaved ends of said         multi-stage dilated optical fibre sections with the core of the         respective first and second ends of said large mode area doped         optical fibre such that a resonant lasing cavity is provided         between said first and second cleaved ends to form an optical         fibre laser.

For lasing to occur, a resonant gain cavity is required in the fibre. This cavity is excited by a pump source and resonance is achieved in the cavity, at the lasing wavelength, using reflectors at either end of the cavity. In a fibre system, the end reflectors are typically fibre Bragg gratings—the most convenient and effective and preferred method. However, further types of reflecting means may be employed, for example, it would be possible to “drill” into the fibre either side of the lasing cavity and deposit silver/gold to achieve end reflection.

Preferably, said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre.

Preferably, said step of processing comprises providing partial end reflectors in both cleaved ends. Preferably, said step of processing comprises inscribing a grating pattern at said cleaved ends. Preferably, said grating pattern comprises a fibre Bragg grating.

The invention further provides an optical fibre laser manufactured according to the above method.

Preferably, the fibre is selected from one of the following types of glass: phosphate, silica, telluride, fluoride, chalcogenide.

Preferably, the fibre is doped with a rare-earth material. Preferably, the fibre is doped with one of the following dopants: erbium, thulium, chromium, ytterbium, neodymium, praseodymium, terbium, or a combination thereof.

It will be understood that the general terms fattening, dilation, expansion and up-tapering are interchangeable, and are used to refer to the expansion of the diameter of an optical fibre as described by the invention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a view of a known optical fibre system;

FIG. 2 is a view of the optical fibre of FIG. 1 after single-stage up-tapering has been performed;

FIG. 3 is a view of the optical fibre of FIG. 1 after two-stage up-tapering has been performed according to the invention;

FIG. 4 shows a series of optical devices that can be manufactured using the up-tapered fibre of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a multi-stage fibre expansion process. For the first-stage fattening, a portion of optical fibre 12 stripped of its buffer 14 (as shown in FIG. 1) is held taut between a pair of vacuum chucks. A heat source, e.g. an oxy-butane flame burner, having a length shorter than that of the optical fibre 12 is applied along a portion of the length of the fibre 12, while a compressive force is applied to the fibre 12.

In general, the heat source may comprise a relatively long flame, the length of which corresponds to the length of that portion of the fibre it is desired to fatten. Alternatively, the heat source may comprise a relatively short flame which is swept back and forth along the length of that portion of the fibre 12.

As the heat source softens the material of the optical fibre, the compression acts to dilate or fatten the body of the fibre 12. This process is repeated until the limitations regarding fibre fattening or up-tapering, e.g. transition losses, start to become significant. In most optical fibre situations, this would approximate to the point where the middle portion of the original fibre 12 has expanded to roughly 2-3 times the original cross-sectional area, as indicated by the section 100 of the fattened fibre 12 a of FIG. 2.

A second-stage fattening is now performed, wherein a heat source is applied along a portion of the expanded section 100 of the fattened fibre 12 a. The heat source for the second stage fattening is applied along a shorter length of the fibre 12 a than the heat source for the first stage, with the result that the second-stage fattening occurs away from the transitions 22 in optical fibre diameter caused as a result of the first-stage fattening. As with the first stage fattening, a compressive force is applied to the fibre 12 a, resulting in the dilation of the heated portion of the fibre 12 a. As with the first fattening stage, the second fattening stage can be performed until the limitations regarding fibre fattening or up-tapering start to become significant, or until the desired dilation is achieved.

With reference to FIG. 3, a second-stage fattened optical fibre is shown at 12 b. As a result of the second fattening stage, the expanded section 100 of the first-stage fattened fibre 12 a has effectively shortened in length to section 100 a, with a second expanded section 102 formed within the boundaries of section 100 a having an increased diameter than that of the fattened section 100 of the first-stage fattened fibre 12 a. As can be seen from FIG. 3, the transitions in diameter 26 between the first-stage fattened fibre 100 a and the second-stage fattened fibre 102 are spaced from the transitions in diameter 22 between the original unfattened optical fibre 12 and the first-stage fattened fibre 100 a. As the fattened sections 100 a,102 are arranged such that a space is maintained between the transitions 22,26, this prevents the occurrence of transition losses at the boundaries due to the adiabatic condition being satisfied.

An indication of the cross-section of the second-stage fattened fibre 12 b along line C-C is indicated at 28. Typical dimensions of the expanded cross-section after up-tapering would be 45 μm diameter for the core 16 and 560 μm diameter for the cladding 18.

Considering an example of a stripped, two-stage fattened fibre, the minimum initial strip length would be approximately 160 mm, and governed by:

-   -   A 5 mm clearance from the stripped buffers 14 at both ends of         the processed fibre;     -   A centred first-stage fattened section length of 15 mm (to         include the second-stage fattened section) up to a diameter of         280 microns; and     -   A centred second-stage fattened section length of 5 mm up to a         diameter of 560 microns.

It will be understood that further fattening stages may be performed as required, in order to further increase the diameter of a fattened fibre, provided that the conditions for fibre fattening are satisfied, e.g. the adiabatic condition. It is predicted that, given the conditions and limits for 2-stage fattening, a 3-stage fattening process may provide a 6-8 times increase in physical diameter from the original fibre size. Preferably, a minimum distance of approximately 5 mm is maintained between successive transition sections.

Up-tapered fibres can then be used in the construction of different fibre optic devices. For example, for an active device, such as an erbium-doped laser, the fattened section provides an interface between a standard fibre and a separate, large mode area, erbium-doped fibre section. For a fibre spectrometer, a cleaved fattened end-face may be processed, for example, by inscribing a pattern, or photo-inducing a pattern, on the end-face using a laser.

Taking the multi-stage fattened fibre 12 b shown in FIG. 3, the fibre 12 b is first cleaved across line C-C, i.e. across the widest cross-section of the fibre 12 b. Once a cleaved multi-stage fattened fibre 12 b is provided, the fibre may be combined with a suitable section of a doped fibre in different manufacturing processes to produce improved optical devices employing multi-stage fattened fibres. Examples of such optical devices can be seen in FIGS. 4( a)-(c).

As the section of the doped fibre (indicated at 30 in FIG. 4) is of a greater diameter than that of the original un-fattened optical fibre 12, the mode-profile is flatter than for an un-fattened fibre with a Gaussian shape mode. A more efficient and more even interaction with the dopants in the fattened optical fibre 30 can therefore be provided in the fibre 30 than for the same length of the original, unfattened fibre 12 that results in a flatter gain response.

Furthermore, the use of particular glass fibres, e.g. a phosphate glass fibre, can allow for a much higher concentration of dopant than, for example, silica glass fibre. Therefore, by selecting a first glass fibre appropriate for the multi-stage fattening (e.g. silica), and splicing the fattened fibre with a glass fibre suitable for high-concentration doping (e.g. phosphate), then an optical device can be manufactured having improved amplification for a relatively small footprint of device.

It will be understood that, while the devices described employ phosphate glass fibres, other types of glass fibres may be used, e.g. phosphate, silica, telluride, fluoride, chalcogenide, bismuth. Similarly, while the examples utilise erbium as the dopant, the fibre section may be doped with any one of the following dopants: erbium, thulium, chromium, ytterbium, praseodymium, neodymium, terbium or a combination thereof.

With reference to FIG. 4( a), an optical wavelength converter is indicated generally at 32. The converter 32 comprises a cleaved end 34 a of the fattened fibre 12 b coupled with the first end 30 a of a length of fibre 30 having an erbium-doped phosphate core 36 with a suitable cladding 38. The fibre 30 has a diameter substantially equal to the diameter of the cleaved end 34 a of the fibre 12 b. The uncleaved, unfattened end of the fibre 12 b is coupled with an optical buffer 14, such that the buffer 14 and the fibre 12 b together form a fattened fibre input launch for the optical wavelength converter 32. The free second end 30 b of the fibre section 30 can be used to provide a wavelength-converted output for a signal input at the optical buffer 14. Such a converter 32 can be used for to provide wavelength conversion having high gain for various purposes, for example, to convert infrared light to visible light for the purposes of charge-coupled device (CCD) detection.

With reference to FIG. 4( b), an optical fibre amplifier is indicated generally at 40. The amplifier 40 comprises a first cleaved end 34 a of the fattened fibre 12 b coupled with the first end 30 a of a length of fibre 30 having an erbium-doped phosphate core 36 with a suitable cladding 38. The second end 30 b of the fibre 30 is coupled with a second cleaved end 34 b of the fattened fibre 12 b.

The respective first and second uncleaved, unfattened ends of the fibre 12 b are indicated at 35 a,35 b in FIG. 4( b). The first cleaved and uncleaved ends 34 a,35 a of the fibre 12 b form a fattened fibre input signal lead and forward pump launch for the optical amplifier 40. Similarly, the second cleaved and uncleaved ends 34 b,35 b form a fattened fibre output signal lead and reverse pump launch for the optical amplifier 40. The optical amplifier 40 shown is suitable for use in systems to amplify optical signals as they propagate along relatively long fibres, e.g. for regeneration purposes.

With reference to FIG. 4( c), an optical fibre laser is indicated generally at 50. The laser 50 may be constructed in a similar fashion to the optical amplifier 40 described above, with the distinction that, prior to the coupling of the cleaved ends 34 a,34 b of the fattened fibre 12 b with the fibre section 30, the cleaved ends 34 a,34 b are processed to provide partial end-reflections at the cleaved ends 34 a,34 b. For lasing to occur in a fibre, a resonant gain cavity is required in the fibre. In general, some form of reflectors are provided at either ends of the resonant gain cavity, the cavity then being excited by a pump source. Resonance is achieved in the cavity by using reflectors operable to reflect light at the lasing wavelength within the cavity. In a fibre system, the end reflectors are typically fibre Bragg gratings—the most convenient and effective and preferred method. However, further types of reflecting means may be employed, for example, it would be possible to “drill” into the fibre either side of the lasing cavity and deposit silver/gold to achieve end reflection. In FIG. 4( c), the reflectors are provided in the form of fibre gratings 52. The gratings 52 provide for the partial reflection of light of a particular wavelength along the fibre section 30. This further processing stage may involve any suitable grating manufacturing process, e.g. inscribing a grating pattern on the fibre, photo-inducing a grating pattern, etc.

The invention is not limited to the embodiments described herein but can be amended or modified without departing from the scope of the present invention. 

1. A fibre dilation method for providing a multi-stage dilated optical fibre, the method comprising the steps of: performing a first-stage fibre dilation process on an optical fibre to form a first dilated section of the optical fibre; and performing a second-stage fibre dilation process on said first dilated section of the optical fibre to form a second dilated section of the optical fibre, wherein said second dilated section is arranged such that the transition in diameter formed between said first dilated section and said second dilated section is spaced from the transition in diameter formed between the undilated section of the optical fibre and said first dilated section wherein said dilation processes comprise: applying a heat source along a portion of the length of an optical fibre to soften said portion of the optical fibre; and applying a compressive force to said portion of the optical fibre to dilate said portion.
 2. (canceled)
 3. The method of claim 1, wherein the method further comprises the steps of iteratively performing at least one successive fibre dilation process on the dilated section of a preceding stage, wherein the transition formed by said at least one successive fibre dilation process is spaced from the transition formed by the preceding stage.
 4. The method of claim 1, wherein the spacing is chosen such that, at any point along the multi-stage dilated optical fibre, the angle of dilation satisfies the following criterion: ${\frac{a}{z}}\frac{a}{z_{b}}$ wherein da/dz defines the tapered angle of dilation, a is the local core radius of the fibre, and z_(b) is the beat length between the HE₁₁ and HE₁₂ modes.
 5. A fibre dilation method for providing a multi-stage dilated optical fibre, the method comprising the steps of: performing a first-stage fibre dilation process on an optical fibre to form a first dilated section of the optical fibre; and performing a second-stage fibre dilation process on said first dilated section of the optical fibre to form a second dilated section of the optical fibre, wherein said second dilated section is arranged such that the transition in diameter formed between said first dilated section and said second dilated section is spaced from the transition in diameter formed between the undilated section of the optical fibre and said first dilated section, wherein the spacing between successive transitions is 5 mm.
 6. The method of claim 1, wherein said first-stage fibre dilation process comprises dilating a section of said optical fibre to approximately 2-3 times the diameter of said optical fibre.
 7. The method of claim 6, wherein said first-stage fibre dilation process comprises dilating a section of said optical fibre to approximately 2.25 times the diameter of said optical fibre.
 8. The method of claim 1, wherein said second-stage fibre dilation process comprises dilating said first dilated section of the optical fibre to approximately 4-5 times the diameter of said optical fibre.
 9. The method of claim 6, wherein said second-stage dilation process comprises dilating said first dilated section of the optical fibre to approximately 4.5 times the diameter of said optical fibre.
 10. A method for coupling standard optical fibres and large mode area fibres, the method comprising the steps of: manufacturing a multi-stage dilated optical fibre according to the method of claim 1; cleaving said multi-stage dilated optical fibre to provide a first cleaved end of a multi-stage dilated optical fibre; providing a large mode area optical fibre having a first end and a second end; and coupling the core of said cleaved end of a multi-stage dilated optical fibre with the first end of the core of said large mode area optical fibre.
 11. A method of manufacture of an optical wavelength converter, the method comprising the steps of: manufacturing a multi-stage dilated optical fibre according to the method of claim 1; cleaving said multi-stage dilated optical fibre to provide a first cleaved end of a multi-stage dilated optical fibre; providing a large mode area doped optical fibre having a first end and a second end; coupling the core of said cleaved end of a multi-stage dilated optical fibre with the first end of the core of said large mode area doped optical fibre, such that said cleaved end of a multi-stage dilated optical fibre and said large mode area doped optical fibre form an optical wavelength converter operable to convert optical signals of a first wavelength received at the uncleaved, undilated end of said multi-stage dilated optical fibre to optical signals of a second wavelength at said second end of said large mode area doped optical fibre.
 12. A method of manufacture of an optical fibre amplifier, the method comprising the steps of: manufacturing a multi-stage dilated optical fibre according to the method of claim 1; cleaving said multi-stage dilated optical fibre to provide a first and a second section of a multi-stage dilated optical fibre having respective first and second cleaved ends and uncleaved ends; providing a large mode area doped optical fibre having a first end and a second end; coupling the core of said first and second cleaved ends of said multi-stage dilated optical fibre sections with the core of the respective first and second ends of said large mode area doped optical fibre; and coupling the first and second uncleaved ends of said multi-stage dilated optical fibre section to form an optical fibre amplifier, the amplifier operable to amplify an optical signal transmitted between said first and second uncleaved ends through said multi-stage dilated optical fibre sections and said large mode area doped optical fibre.
 13. A method of manufacture of an optical fibre laser, the method comprising the steps of: manufacturing a multi-stage dilated optical fibre according to the method of claim 1; cleaving said multi-stage dilated optical fibre to provide a first and a second section of a multi-stage dilated optical fibre having respective first and second cleaved ends and uncleaved ends; processing said first and second cleaved ends to provide reflecting means at said cleaved ends; providing a large mode area doped optical fibre having a first end and a second end; coupling the core of said first and second cleaved ends of said multi-stage dilated optical fibre sections with the core of the respective first and second ends of said large mode area doped optical fibre such that a resonant lasing cavity is provided between said first and second cleaved ends to form an optical fibre laser.
 14. The method of claim 13, wherein said step of processing comprises one of: inscribing a grating pattern on said at least one cleaved end; or photo-inducing a grating pattern on said at least one cleaved end.
 15. The method of claim 10, wherein said step of cleaving the multi-stage dilated optical fibre comprises cleaving the fibre across the widest cross-section of said fibre.
 16. The method of claim 10, further comprising the step of selecting a large mode area doped optical fibre such that the mode diameter of the core of said large mode area doped optical fibre substantially corresponds to the mode diameter of the core of said cleaved end.
 17. The method of claim 10, wherein the large mode area doped optical fibre is selected from one of the following types of glass: phosphate, silica, telluride, fluoride, chalcogenide, bismuth.
 18. The method of claim 10, wherein the large mode area doped optical fibre is doped with a rare-earth material.
 19. The method of claim 18, wherein the large mode area doped optical fibre is doped with one of the following dopants: erbium, thulium, chromium, ytterbium, neodymium, praseodymium, terbium or a combination thereof.
 20. An optical wavelength converter manufactured according to the method of claim
 11. 21. An optical fibre amplifier manufactured according to the method of claim
 12. 22. An optical fibre laser manufactured according to the method of claim
 14. 23. A fibre comprising a portion of standard optical fibre and a portion of large mode area optical fibre coupled according to the method of claim
 10. 24. A multi-stage dilated optical fibre manufactured according to the method of claim
 1. 